Chemical ordering and composition fluctuations at the (001) surface of theFe64Ni36Invar alloy

Abstract

We report on a study of (001) oriented fcc Fe-Ni alloy surfaces which combines first-principles calculations and low-temperature STM experiments. Density functional theory calculations show that Fe-Ni alloy surfaces are buckled with the Fe atoms slightly shifted outwards and the Ni atoms inwards. This is consistent with the observation that the atoms in the surface layer can be chemically distinguished in the STM image: brighter spots (corrugation maxima with increased apparent height) indicate iron atoms, darker ones nickel atoms. This chemical contrast reveals a c(2×2) chemical order (50% Fe) with frequent Fe-rich defects on the Fe64Ni36(001) surface. The calculations also indicate that subsurface composition fluctuations may additionally modulate the apparent height of the surface atoms. The STM images show that this effect is pronounced compared to the surfaces of other disordered alloys, which suggests that some chemical order and corresponding concentration fluctuations exist also in the subsurface layers of Invar alloy. In addition, detailed electronic structure calculations allow us to identify the nature of a distinct peak below the Fermi level observed in the tunneling spectra. This peak corresponds to a surface resonance band which is particularly pronounced in iron-rich surface regions and provides a second type of chemical contrast with less spatial resolution but one that is essentially independent of the subsurface composition.

Fe–Ni Surface Alloy Formation on Ni(111) Investigated by Scanning Tunneling Microscopy

“…2). The dark spots are similar to the Ni atoms on the Fe-Ni alloy observed by Ondráček et al 12) and are distributed randomly on the surface with about 41% coverage. The AES spectra shows the marked decrease of Fe from the surface (Fig.…”

“…They found that the Fe atoms appear as bright spots while the Ni atoms appear as dark spots on the Fe-Ni alloy surface. 12) In the present study, we observed that the elongated bcc-Fe domains disappear and a triangular network structure and large accumulated bcc-Fe islands are formed on the surface after heating at 550 K [ Fig. 2(a)].…”

“…4,5) The Fe-Ni alloys were able to exist, in all compositions, in a chemically disordered form like a solid solution in which Fe and Ni atoms occupy crystal lattice sites randomly. 10) The ordered phases were restricted to FeNi (tetrataenite) and FeNi 3 compositions 10,11) and their ordering temperatures were lower than 670 K. 11) Recently, Ondráček et al 12) observed the (001) surface of the Fe 64 Ni 34 Invar alloy by STM. They found that the Fe atoms appear as bright spots while the Ni atoms appear as dark spots on the Fe-Ni alloy surface.…”

Reactive metal–oxide interfaces: A microscopic view

“…25 Fe atoms is clearly visible (see Section 4.1), suggesting that the surface layer can be regarded as a Ni x Fe y O single layer. This interpretation is also supported by the presence of a strong peak in STS spectra detected at about 0.25 eV below E F , very similar to the resonance observed in spectra acquired on invar alloys [259]. 1 4.3.…”

Noncollinear Surface Spin Density by Surface Reconstruction in the Alloy NiMn

“…At the center of the Brillouin zone, the local density of states decays slowest towards the vacuum and contributes presumably largest to the tunnel current [20]. By assuming the magnetization perpendicular to the surface, it is found that, without relaxation or without spin-orbit interaction, the spin polarizationP of the surface atoms is fully aligned with the magnetization direction, thus forming a collinear spin structure.…”